SUMMARY: The distal nephron of the kidney has essential functions for urine concentration and electrolyte homeostasis. Defects in specific segments of the distal nephron can cause various kidney diseases, such as salt-losing tubulopathiesthatarecharacterizedbyaninabilitytoconcentrateurine.Allnephronsareformedinthemouse by ~P3 postnatally, but the full functional capacity of the distal nephron is only achieved after a subsequent maturation phase, during which the distal nephron undergoes significant growth and functional changes. The maturationofthedistalnephronhasremainedlargelyunstudiedanditisnotknownwhichmolecularandcellular mechanisms drive distal nephron maturation. Here, we will investigate the molecular mechanisms that orchestratedistalnephronmaturation. We have identified the BTB-domain containing nuclear protein KCTD1, a transcriptional repressor, as an essentialregulatorofdistalnephronmaturation,forwhichafunctioninthekidneywaspreviouslyunknown.We found KCTD1 to be expressed only in the distal nephron epithelium in the kidney andshow that its deficiency impairsmaturationandfunctionofthethickascendinglimbofHenleandthedistalconvolutedtubule,resulting in an early-onset salt-losing tubulopathy with a diminishedability to concentrate urine. The immature tubules undergoprogressivedilatationandformenlargingcysts,changesleadingtolate-onsetkidneyfibrosisandrenal failure.InducibleinactivationofKCTD1duringthedistalnephronmaturationphaseleadstothesedistaltubule defects, but not when KCTD1 is inactivated in fully matured kidneys. Importantly, we identified missense mutationsinKCTD1inpatientsthatdevelopkidneyabnormalitiesresemblingthefindingsinKCTD1nullmice, establishingtheclinicalrelevanceofKCTD1forhumankidneyfunctions.Mechanistically,weshowthatlossof KCTD1leadstopostnatalderepressionofthenuclearproteinDAPL1whichprecedesthemanifestationofthe distal nephron maturation defect and is associated with loss of the Wnt/planar cell polarity protein Dvl2 and increasedcanonicalWnt/?-cateninsignaling.Furthermore,weshowthatDAPL1regulatesprimaryhumandistal nephron epithelial cell differentiation. Here, we will investigate the molecular mechanisms of how KCTD1 and DAPL1 regulate distal nephron maturation. Our proposed experiments are highly innovative and have significantclinicalrelevance,astheyaimtouncoverfundamentalnewmechanismsthatarerequiredfordistal nephron maturation and which are impaired in several kidney disorders. Moreover, we will investigate how defectsindistalnephronmaturationaffectlate-onsetchronickidneydisease-likepathologiesandcystickidney disease.Thescientificpremiseforthisapplicationisstrongandbuildsonextensivepreliminarydata,mouse modelsthathavealreadybeenestablishedinourlaboratory,andaprimaryhumanTAL/DCTcellculturesystem forwhichthefeasibilityoftheproposedexperimentshasbeenclearlyestablished.